This invention relates to the control of nuclear reactors and more particularly to the closed-loop control of reactor power.
To change the steady state power level of a nuclear reactor, neutron-absorbing control elements such as control rods or shim blades are moved in and out of the reactor core. Also, certain soluble chemicals may be used as the control element. For example, to increase the power of a nuclear reactor, the control rods are moved outwardly so that the neutron population can build up to a level consistent with the higher output level. Once the population has increased to the suitable level, the control rods are reinserted to their original position less the impact of feedback effects such as temperature and voids on the reactor's neutron population. Alternatively, if chemical poisons were being used, the concentration of the chemical would be decreased, the power allowed to rise, and the concentration then restored to its initial value less feedback effects. The reactor will then continue to operate at the new higher output level. Because of the nuclear reactor dynamics to be discussed in more detail below, the direction of control rod movement or the change in concentration of the chemical, such as boric acid, must be reversed before the new power level has been attained. Normally, this reversal is not accomplished as one continuous movement but in a series of short reversals interrupted by intervals in which the rod position or chemical concentration is kept constant. Once the reversal in the direction of rod travel or the change in chemical concentration has been initiated, the power may continue to build up but at a slower rate. If the change was performed properly, the power level will climb to the desired new operating level without overshoot or undershoot. At the present time decisions concerning control mechanism reversal are made by the licensed reactor operator based on knowledge of the dynamics of the reactor and the experience of the operator. These decisions are complicated because (1) the equations of reactor dynamics are non-linear, (2) the rate of change of power depends on both the net change in the control mechanism's position or concentration and its rate of change, (3) there are feedback effects between the reactor power and the rate at which power is changing, and (4) control mechanisms have non-linear strengths and finite speeds.
It is therefore an object of this invention to provide a closed-loop control system for regulating reactor power in a nuclear reactor.
It is a further object of the invention to provide such a closed-loop system which results in the attainment of new power levels without overshoot (or conversely, undershoot) beyond that allowed by specified tolerance bands.
It is yet another object of the invention to provide such a closed-loop controller which at all times restricts the net reactivity so that it is always possible to rapidly make the reactor period infinite whenever required.
Still another object of the invention is a closed-loop nuclear reactor control scheme that guarantees that no action initiated by any automatic control law will ever result in a challenge to the existing nuclear safety system provided that the decision of the control law is subject to review by the control scheme.
Yet another object of the invention is a closed-loop power controller which recognizes that reactor dynamics are non-linear, that the rate of change of reactor power depends both on the net reactivity and the rate of change reactivity, that the reactivity is dependent on the reactor power through various feedback mechanisms, and that control mechanisms have finite speeds as well as position-dependent, non-linear worths.